This invention relates to a blended composition containing polyacetal resin (also referred to herein as polyoxymethylene), which composition displays a toughness superior to that of polyacetal but does not sacrifice the other desirable properties that are inherent to a polyacetal resin. More particularly, this invention provides a polyacetal resin composition comprising a small amount of a thermoplastic polyurethane, which composition is manufactured by preparing a polyacetal masterbatch component in which a thermoplastic polyurethane is present at a proportionately high level of content, followed by blending this masterbatch component with further polyacetal resin to dilute the proportionate amount of polyurethane in the final blended composition to the desired level.
Polyacetal resin, which is prepared by polymerizing a starting material such as formaldehyde monomer or trioxane (which is a trimer of formaldehyde) exhibits excellent mechanical and physical properties, such as tensile strength, stiffness, fatigue resistance, sliding resistance, chemical resistance, and the like. Comonomers may also be present. It is extensively used in mechanical parts, electronic parts, automotive parts, and the like. For certain molded articles, however, obtained by molding a conventional composition containing essentially only polyacetal resin, it would be desirable to have greater toughness than has heretofore been possible with such conventional compositions. Improving the toughness of a composition of a polyacetal resin has previously required the addition of substantial amounts of toughening agent(s) into the composition. The addition of a substantial amount of toughening agent(s) into a polyacetal composition has often been disfavored, however, since the stiffness and moldability of the composition will be adversely affected, thereby making molding difficult, and disadvantageous effects on operativity and manufacturing costs will also result.
Many technologies related to the improvement of impact resistance by the addition of a variety of additives into a polyacetal resin composition have been known. For example, the prior art technologies for improving impact resistance involve blending a toughening agent, such as polyurethane, into polyoxymethylene [Japanese laid-open patent No. 59 (84)-155452, Japanese laid-open patent No. 59 (84)-155453 and Japanese laid-open patent No. 61 (86)-19652].
U.S. Pat. No. 5,286,807 discloses polyoxymethylene compositions having 5-15 wt % thermoplastic polyurethane which has a soft segment glass transition of lower than 0xc2x0 C. and is dispersed in the polyoxymethylene as discrete particles in order to achieve exraordinary impact resistance as measured by Gardner impact.
U.S. Pat. No. 4,804,716 discloses polyoxymethylene compositions having 15-40 wt % thermoplastic polyurethane which has a soft segment glass transition of lower than xe2x88x9215xc2x0 C. and is dispersed in the polyoxymethylene as discrete particles in order to achieve extraordinary impact resistance as measured by Gardner impact.
Japanese laid-open patent No. 4-198355 discloses a polyoxymethylene resin composition comprising 100 wt % polyoxymethylene resin and 1-150 wt % thermoplastic polyurethane to provide molded articles having excellent impact resistance and mechanical properties and good in surface appearance and heat aging resistance.
Japanese laid-open patent No. 7-207115 discloses a polyacetal resin composition comprising a thermoplastic elastomer, preferably a polyurethane or a polyolefin elastomer having 1-3000 kg/cm2 elastic modulus, which is dispersed in the form of particles with average distance between the particles being given by a specified formula in order to improve impact resistance.
Japanese laid-open patent No. 5-262957 discloses a polyacetal resin composition comprising 99-40 wt % polyacetal, 1-60 wt % polyester type thermoplastic polyurethane elastomer and 0.01-10 wt % polyhydric alcohol having 3 hydroxyl group in the molecule, and being prepared by melt-mixing these components at a temperature in the range of 180 to 250xc2x0 C. under shearing condition.
Despite the introduction in the art of the compositions described above, there is still a demand for means to improve the toughness of polyacetal in a composition without adversely affecting the other inherent properties of the polyacetal resin. When preparing a blended composition from polyacetal and polyurethane, it would also be desirable to do so in an manner that suppresses any significant degradation of the polyurethane, and any polyacetal yellowing, resulting from the presence of the polyurethane at the higher temperatures encountered during processing and molding.
An extensive study by the present inventors has led to the blended composition of this invention, in which greater toughness is obtained than is characteristic of polyacetal alone, but in which there is no offsetting reduction of the other desirable properties that are inherent to the polyacetal resin.
In one aspect, this invention involves a blended composition comprising in admixture (a) a polyacetal masterbatch component, and (b) a polyacetal dilution component.
In another aspect this invention involves a process for making a blended composition comprising the steps of (a) preparing a polyacetal masterbatch component from a polyacetal resin and a thermoplastic polyurethane, (b) admixing the polyacetal masterbatch component with a polyacetal dilution component, and (c) recovering a blended composition.
In yet another aspect, this invention involves a toughened composition comprising (a) a polyacetal resin, and (b) a thermoplastic polyurethane containing a soft segment having a glass transition temperature of less than xe2x88x9230xc2x0 C., wherein
(i) said toughened composition is prepared by mixing a polyacetal masterbatch component with a polyacetal dilution component,
(ii) said polyacetal masterbatch component is prepared by mixing a polyacetal resin with a thermoplastic polyurethane, said thermoplastic polyurethane being about 10 to about 60 wt % of the masterbatch component, and
(iii) said toughened composition has a total thermoplastic polyurethane content of about 1 to about 15 wt %.
In a further aspect, this invention involves an article of manufacture prepared from the blended composition of this invention.